In general, a CCD is mainly used as an image sensor for a facsimile, a copier, a digital camera, etc., but in recent years, an amplification type photoelectric conversion device that includes an amplifying element such as a MOS transistor or a bipolar transistor in each pixel unit has been used. In an amplification type photoelectric conversion device, it is important to remove noise in order to read a signal with high sensitivity.
FIG. 6 shows an example of an amplification type photoelectric conversion device according to the related art. In a photoelectric conversion element 101, even though light is not incident thereon, current flows, which generates noises. Actually, in a case of reading a signal generated by light that is incident onto the photoelectric conversion device 101, it is necessary to remove dark voltage noise and noise caused due to variation of a dark voltage (hereinafter, referred to as a noise signal component). First, in order to output a noise signal component in a case in which light is not incident onto the photoelectric conversion element 101, a selecting unit, which is composed of a MOS transistor 104, is turned on, and through a buffer amplifier 103, a noise signal component, such as electric charge, is accumulated in a noise accumulating unit 106. After elapse of arbitrary accumulation time, the MOS transistor 104 is turned off. Next, the signal generated by the incident light onto the photoelectric conversion element 101 passes through the buffer amplifier 103 while MOS transistor 105 is turned on; thereby charge is accumulated in a signal accumulating unit 107. After an elapse of arbitrary accumulation time, the MOS transistor 105 is turned off.
At this time, electric charge owing to dark voltage noise and electric charge when light is incident are accumulated in the accumulating units 106 and 107, respectively. Then, MOS transistors 110 and 111 are simultaneously turned on and thus the signal components are input to a differential amplifier 114 through buffer amplifiers 112 and 113, respectively. The differential amplifier 114 obtains the difference between the signal components from the accumulating units 107 and 106, and obtains the signal occurring due to a real incident light by removing the noise signal component of the accumulating unit 106. Finally, MOS transistors 108 and 109 are turned on such that charge in the accumulating units 106 and 107 is reset. Further, in order to refresh the remaining charge of the photoelectric conversion element 101, a MOS transistor 102 is turned on such that an offset voltage is applied. In this way, it is possible to remove the noise signal component and to exactly obtain a net signal due to real incident light (refer to, for example, JP-A-9-205588 and JP-A-8-255027).
However, the differential amplifier 114 requires various elements (in particular, at least, eight MOS transistors, one capacitor, and one resistor) as shown in FIG. 5. Therefore, there is a problem in that the area occupied by the differential amplifier does not reduce as the size of the photoelectric conversion device reduces. Further, two buffer amplifiers, that is, the buffer amplifier 112 for dark voltage noise and the buffer amplifier 113 for an optical signal are required. This is a problem in terms of the occupied area. Similar to the differential amplifier, the buffer amplifiers 112 and 113 also generally include a plurality of MOS transistors. Further, in order to make the buffer amplifiers or the differential amplifier function generally with high accuracy, there are cases of using even more elements, whereby it is difficult to disregard the area problem.
Furthermore, variation of an output voltage occurs between two amplifiers, that is, the buffer amplifier 112 for dark voltage noise and the buffer amplifier 113 for an optical signal. Therefore, actually, there is a possibility that the dark voltage noise signal component is not completely removed and thus an exact signal is not read. In particular, the buffer amplifiers 112 and 113 are amplifiers having a feedback amplification factor of 1 and include a plurality of MOS transistors or bipolar transistors. Therefore, property error and deviation of a transistor element due to a manufacturing process of a semiconductor integrated circuit or the like may occur and in particular, the possibility that variation of an output voltage occurs tends to increase as the number of elements increases.
The present invention has been finalized in view of the above-mentioned drawbacks in the related art, and it is an object of the invention to provide a photoelectric conversion device that removes a noise signal component and reads a signal due to incident light, and that reduces the cost and occupied area and removes variation among a plurality of buffer amplifiers by simplifying the system configuration.